The present invention is concerned with neutralising space charge in ion beams travelling through regions of applied magnetic field, and in particular, although not exclusively, with neutralising space charge in an ion beam as it travels through the flight tube of an analysing magnet.
An analysing magnet generates a substantially uniform magnetic field in its flight tube, causing an ion travelling through the flight tube to follow a curved path in a plane perpendicular to the direction of the magnetic field. The radius of the curved path is given by:
r=mv/qB=(2Em)xc2xd/qB
where v,E,m and q respectively are the velocity, kinetic energy, mass and charge of the ion, and B is the magnitude of the magnetic flux density in the flight tube. The analysing magnet can therefore be used to resolve spatially (in a dispersion plane perpendicular to the magnetic field in the flight tube), ions in a beam according to their energy, mass and charge.
In ion implanters, an analysing magnet is used in conjunction with a selection slit to select ions of the required species from an incident beam for implantation in a target semiconductor substrate. Typically, the incident beam will comprise ions having substantially the same energy, and the magnet is arranged to focus those ions having the desired mass/charge ratio at the selection slit so that only they pass through the slit and go on to impinge on the target.
In spectrometry applications, analysing magnets are used to resolve ions in a beam according to their mass, energy and charge for separate detection.
Ideally, for both ion implantation and spectrometry applications, ions in the beam entering the flight tube having the same energy, charge and mass should all be focused by the analysing magnet onto a common line, perpendicular to the dispersion plane, as they exit.
However, in the absence of any neutralising effect, a beam containing only ions of a particular polarity will experience space charge effects. The mutual repulsion of the ions in the beam tends to cause the beam to diverge or xe2x80x9cblow upxe2x80x9d. This mutual repulsion means that the position at which an ion exits the magnet is no longer solely determined by its incident velocity, mass and charge, and the applied magnetic field.
In ion implantation applications, although an incident ion may have the desired mass/charge ratio, because of space charge effects it may not be focused on the selection slit and so may not reach the target. This reduces the ion implantation current reaching the target from a given source and increases the process time required to achieve a desired implantation dose. In addition, space charge effects may result in an increased number of incident beam ions hitting the sides of the flight tube. This further reduces the implantation current reaching the target and can result in contamination of the target by particles sputtered off the flight tube.
Similarly, in spectrometry applications, beam xe2x80x9cblow upxe2x80x9d inhibits the spatial resolution of different ions and reduces signal intensity.
In applications where a scanning magnet is used to deflect an ion beam (for example to scan the beam across a target) beam blow up inside the magnet is also undesirable. It reduces beam intensity and control accuracy.
The effect of beam space charge is especially severe for relatively low energy beams (e.g. 1-2 keV)since for the same beam current there is a higher density of ions in a low energy beam.
In regions of zero electric field, such as the flight tube of an analysing or scanning magnet, self neutralisation of ion beams tends to occur through the production of electrons and positive ions resulting from collisions between beam ions and atoms of residual gas in the vacuum chamber through which the beam is passing. However, this self neutralisation may be insufficient adequately to reduce beam blow up. This is particularly true for relatively low energy ion beams, as, at low energies, the cross sections for electron production during interaction between the beam ions and residual gas atoms are extremely small.
Also, some self neutralisation may occur as a result of local electron production from the beam striking the inside of the flight tube along the entire beam path through the magnet. Again however, this may be insufficient to reduce beam blow up to acceptably low levels, and in any case is to be avoided as it results in beam loss and unwanted particle generation.
In regions of zero electric field and zero magnetic field (i.e. xe2x80x9cdrift spacexe2x80x9d) a known technique to neutralise space charge is to flood the region through which the ion beam travels with low energy (typically a few eV) electrons or ions, produced, for example, in a plasma chamber adjacent to the beam flight path. In this drift space the electrons or ions are mobile and can move along and across the beam to minimise beam potential.
In regions of applied magnetic field however, the magnetic field severely limits the mobility of these electrons or ions. In applied fields of sufficient magnitude to deflect beams of ions with energies in excess of, say, a few keV, low energy charged particles, and electrons in particular (with their small mass,) will follow paths having circular projections of very small radii on a plane perpendicular to the direction of applied field. In effect, the electrons are restricted just to following the magnetic field lines. They have substantially zero mobility perpendicular to the applied field, which in the case of analysing magnets or scanning magnets means that the electrons have substantially zero mobility along the beam axis.
In regions where the electric field is nominally zero, such as inside the earthed flight tube of an analysing magnet, there may in fact be a small amount of electron motion perpendicular to the direction of applied magnetic field owing to the presence of various small electric fields such as those resulting from the beam itself. This motion is known as Exc3x97B motion. However, in such nominally E-field-free regions, electron mobility along the beam is, in general, very restricted.
Thus it is not possible to neutralise beam space charge inside regions of applied field by introducing low energy charged particles to adjacent drift space regions as the charged particles will not be able to migrate into the beam.
According to a first aspect of the present invention there is provided ion beam apparatus comprising:
a analysing magnet including a flight tube for receiving and conveying through the magnet a beam of ions, the magnet being operable to generate a substantially uniform magnetic field in the flight tube to deflect beam ions according to their mass/charge ratio in a dispersion plane perpendicular to the direction of said uniform magnetic field; and
a thermionic electron source inside the flight tube, arranged adjacent and outside a nominal cross section of a beam of ions travelling through the magnet and extending along a nominal flight path of a beam travelling through the magnet,
the thermionic electron source being further arranged such that the projection of the thermionic electron source on the dispersion plane and the nominal projection on the dispersion plane of an ion beam travelling through the magnet overlap at a plurality of positions along the nominal flight path of the beam.
Thus, magnetic flux generated by the magnet may link the thermionic electron source and a beam of ions travelling through the magnet at a plurality of positions along the flight path, and electrons may be emitted thermionically from the source into the beam at these positions.
This enables space charge to be neutralised at a plurality of positions along the beam in spite of low electron mobility along the beam, and so reduces xe2x80x9cblow upxe2x80x9d, keeping the beam tight. The thermionic electron source is positioned outside the nominal beam envelope to reduce beam contamination resulting from sputtering off the source and the associated erosion of the source.
In the presence of the magnetic field generated by the analysing magnet, electrons emitted from the thermionic electron source will simply follow field lines passing through the source. These field lines will be perpendicular to the dispersion plane of the magnet. Only those emitted electrons travelling along field lines which also link the ion beam will, therefore, enter the beam and contribute to neutralisation of its space charge (either directly, in the case of a beam of positive ions, or, in the case of a beam of negative ions, by colliding with residual or deliberately introduced gas atoms or molecules to produce positive ions).
Clearly, the greater the degree of overlap between the projections on the dispersion plane of the thermionic electron source and the ion beam, the greater the proportion of electrons emitted from the source that are able to enter the beam.
Advantageously, the thermionic electron source may extend substantially in a plane spaced from and parallel to the dispersion plane. In an analysing magnet with a nominally horizontal dispersion plane, the thermionic electron source may therefore be arranged in a plane just above, or just below the nominal beam cross section.
In a preferred embodiment the thermionic electron source is positioned as close as possible to the nominal beam cross section to maximise the proportion of the flight tube cross section available for beam transport.
In certain embodiments, the thermionic electron source comprises a longitudinal electrically conductive filament running along the nominal flight path, above and substantially parallel to the nominal centre of an ion beam travelling through the analysing magnet. The filament may therefore be linked to the beam by magnetic flux along a substantial part of its length, and so can be used to emit electrons thermionically to neutralise space charge along a large fraction of the beam""s path length through the magnet.
A plurality of spaced-apart longitudinal filaments may be used to neutralise space charge both along the length and across the width of a beam.
In a further preferred embodiment, the thermionic electron source comprises a plurality of transverse filaments, spaced along the flight path, and each extending across the nominal beam width.
The transverse filaments may for example, be connected to bus bars at opposite sides of the beam, or may be in the form of hairpins, connected to two separate bus bars at the same side of the beam. Hairpin geometry provides the advantage that the filaments are less prone to breaking as a result of relative movement of the bus bars, or thermal expansion or contraction.
The transverse filaments provide the advantage of enabling space charge to be neutralised across the beam at a plurality of locations along its path through the magnet.
The thermionic electron source may comprise an array or grid of filaments, which may be planar and, for example, be machined or cut from a graphite sheet.
Heat shields may be incorporated between the thermionic electron source and the inside of the flight tube to reduce the power needed to maintain the source at a temperature sufficient to yield a desired emission current.
A wide variety of materials may be used for the filament or filaments,including, for example, tungsten.
In a preferred embodiment however, graphite filaments are employed. For ion implantation applications in particular, graphite filaments are desirable because carbon sputtered off the filaments as a result of beam strike is a more tolerable beam contaminant than are metal atoms or ions.
In cases where the beam travelling through the magnet comprises just positive ions, the thermionically emitted electrons may neutralise the beam space charge directly. The electrons travel along flux lines into the beam, and once inside contribute to space charge neutralisation.
In the case of beams comprising negative ions travelling through the magnets, thermionic electrons may be used to ionise residual gas atoms or molecules in the partial vacuum of the flight tube. The positive ions produced by collisions between the thermionic electrons and the residual atoms or molecules may then contribute to the neutralisation of beam space charge.
To facilitate the neutralisation of negative ion beams, charge-neutral atoms or molecules may deliberately be introduced into the flight tube for ionisation by thermionically emitted electrons.
A thermionically emitted electron entering the beam will, in general, only remain in the beam if it suffers a collision inside the beam, and as a result loses energy. Otherwise, it will have sufficient energy to escape any beam potential well and will cease to contribute to beam space charge neutralisation. Collisions with ions or neutral atoms may be elastic or inelastic. At very low energies, below the thresholds for excitation and ionisation, elastic collisions, which involve very small energy transfers, dominate and the electrons may simply bounce off their collision partners and escape from the beam. Collisions with other electrons would involve greater energy transfer, but the collision cross-section is very small. The inelastic collision cross-sections can be increased significantly by increasing the electron energy by applying a negative bias to the thermionic electron source. In preferred embodiments therefore, the thermionic electron source is negatively biased with respect to the flight tube, and this bias voltage may be adjusted to control the energy of the emitted electrons.
This bias enables the injection of electrons into the beam to be controlled and also provides the energy required to ionise the background (i.e. residual) gas which is important in the case of the space charge neutralisation of negative ion beams. Control of electron emission energy is also important in the case of positive ion beams in that slow positive ions produced by collisions between residual, or deliberately introduced, neutral gas atoms or molecules and low energy emitted electrons are an important feature of space charge neutralization because they help to trap electrons. If slow negative ions are produced, an unlikely process in most gases, they too would assist in the neutralization of the positive ion space charge.
A beam of positive ions may be surrounded by a sheath of negative space charge which provides a potential barrier to the entry of thermionically emitted electrons into the core of the beam. Biasing the source can provide the emitted electrons with sufficient energy to penetrate this barrier.
Depending on the particular application, optimum biases in the range 3V to70V have been found, although optimum values outside this range may be expected.
Advantageously, one or more electron repellers, may be arranged inside the flight tube and outside the nominal beam envelope to reflect electrons escaping the beam back into the beam. The electron repellers are negatively biased with respect to the thermionic source, and the bias may be adjustable. In one application, a weak optimum bias of 3V was found. The reflectors may be positioned on opposite sides of the beam, and enable the lifetime of electrons in the beam volume to be extended. Energetic electrons which, in the absence of collisions, would not remain in the beam, may be reflected back and forth substantially along field lines linking opposing repellers. The xe2x80x9cconfinementxe2x80x9d of electrons between opposing repellers is possible because the mobility of electrons in directions perpendicular to the applied magnetic field is severely restricted.
The repellers may be planar, and may comprise graphite sheets. The repellers may in fact also function as heat shields.
According to a second aspect of the present invention there is provided ion beam apparatus comprising:
an analysing magnet including a flight tube for receiving and conveying through the magnet a beam of ions, the magnet being operable to generate a substantially uniform magnetic field in the flight tube to deflect beam ions according to their mass/charge ratio in a dispersion plane perpendicular to the direction of said uniform magnetic field; and
a thermionic electron source inside the flight tube, extending along the nominal flight path of said beam and being arranged inside the nominal envelope of said beam.
The term xe2x80x9cenvelopexe2x80x9d is used to denote the three-dimensional surface nominally bounding the beam, i.e. it represents the nominal extent of the beam. An object placed inside the nominal beam envelope will, therefore, nominally be struck by the beam. An object inside the nominal beam envelope is in the nominal path of the beam.
By positioning the source at least partially inside the beam, low energy electrons may be emitted which have insufficient energy to escape the beam potential well. These electrons may therefore remain in the beam for a significant time and contribute to space charge neutralisation. Again, the emitted electron energy may be controlled by varying the bias on the source with respect to the flight tube.
The source inside the beam may comprise an array of filaments, which may be planar or may follow a curved surface including the nominal beam centre line.
The array may extend over at least part of the height and/or the width of the beam to provide more homogeneous space charge neutralisation.
The array may comprise straight, curved, hair-pin or other geometry filaments, and may be a grid.
The source may be electrically conductive, may comprise carbon, and/or may comprise refractory material. The source may be arranged to be heated electrically, or by the beam, or both.
In order to reduce contamination of the beam conveyed to the remainder of the apparatus, downstream of the magnet, the parts of the thermionic electron source inside the nominal beam envelope (i.e. these parts nominally subjected to beam strike) may be arranged to be out of line of sight with the selection slit at the exit of the flight tube.
According to a third aspect of the present invention there is provided ion beam apparatus comprising:
a magnet arranged to generate a magnetic field in a flight tube to deflect a beam of ions travelling through said flight tube; and
a thermionic electron source inside said flight tube, extending along the nominal flight path of said beam and arranged to emit electrons thermionically into said beam.
The source may be arranged outside the nominal beam envelope and further arranged to be linked to the beam at a plurality of positions along its path by magnetic flux generated by the magnet, or the source may be arranged at least partially inside the nominal beam envelope. The source may comprise an array of filaments, and the apparatus may further comprise electron repellers, heat shields, and means for biasing the source and the repellers.
According to a fourth aspect of the present invention there is provided a method of neutralising space charge in a beam of ions travelling in a flight tube through a magnet, the method comprising the steps of:
providing a source of thermionic electrons inside the flight tube, adjacent and outside the nominal cross section of the beam and extending along the nominal flight path of the beam;
arranging the thermionic electron source to be linked to the beam at a plurality of positions along the nominal flight path by magnetic flux generated by the magnet; and
emitting electrons thermionically from the thermionic electron source.
According to a fifth aspect of the present invention there is provided a method of neutralising space charge in a beam of ions travelling in a flight tube through a magnet, the method comprising the steps of:
providing a source of thermionic electrons inside the beam, said source extending along the beam; and emitting electrons thermionically from said source.
The magnet may, for example, be an analysing magnet, a scanning magnet, or a magnet arranged to deflect the beam for other purposes. It will be apparent that the term xe2x80x9cflight tubexe2x80x9d is used simply to denote the region of space inside the magnet through which ion beam travels. The xe2x80x9cflight tubexe2x80x9d in general need have no sides as such, but typically comprises an electrically conductive tube inside which the electric field is arranged to be zero.